21 Boiler Fault Code: What It Means on Every Brand

The 21 fault code appears on 2 boiler brands — Intergas and Johnson & Starley — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What 21 means	Severity	DIY?
IIntergas	Fault code 21 on Intergas boilers indicates a problem with the S2 temperature sensor — the return-water sensor built into Intergas's distinctive two-in-one heat exchanger. Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	Fault code 21 on Johnson & Starley warm air units points to a problem with the Air Circulation Fan (ACF) — the fan responsible for pushing heated air through the duct system and into each… Full guide →	Lockout — engineer likely	No

21 brand by brand

Intergas 21

Fault code 21 on Intergas boilers indicates a problem with the S2 temperature sensor — the return-water sensor built into Intergas's distinctive two-in-one heat exchanger. This sensor continuously monitors the temperature of water coming back from the heating circuit into the boiler, and the control board uses that reading to manage burner output safely. When the signal from S2 falls outside the expected range — or disappears entirely — the boiler locks out and shows code 21 as a protective measure. Codes 20, 21, 22, 23, and 24 all relate to the same S2 sensor circuit; they represent slightly different failure modes (for example, an open-circuit reading, a short-circuit reading, or a value that is implausibly high or low) but all point to the same root cause. If your display shows any code in the 20–24 range, the guidance on this page applies.

Causes, fixes & costs for Intergas 21 → · All Intergas codes

Johnson & Starley 21

Fault code 21 on Johnson & Starley warm air units points to a problem with the Air Circulation Fan (ACF) — the fan responsible for pushing heated air through the duct system and into each room. The boiler's control system has detected that this fan is either not running at all, or is failing to reach the required maximum speed. On WarmCair condensing warm air models (such as the C10D, C16D, C26D, and C36U), this is a diagnosed ACF defect as defined in the manufacturer's own service documentation. On older Economaire warm air units, a code 21 is treated as a serious unresolved fault, and even trained engineers are directed to contact Johnson & Starley directly before proceeding — it is not a fault with a simple off-the-shelf fix. In both cases, this is not something a homeowner can safely address and specialist warm air engineer attendance is essential.

Causes, fixes & costs for Johnson & Starley 21 → · All Johnson & Starley codes

Frequently asked questions

Is the 21 fault code the same on every boiler?

Broadly, yes — on Intergas and Johnson & Starley the 21 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a 21 fault myself?

Not usually. On every brand listed, 21 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a 21 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.